Generally, the hydraulic pump motors used for power dividing type hydraulic transmission systems are restricted in their size when they are used in automobiles, so that such motors are subject to certain limitations in use at high speeds. In order to obtain a widely variable speed range with the small-sized hydraulic pump motors at low-speed rotations, there has been proposed a so-called mode conversion system in which the speed ratio is changed by changing the engagement of the clutches from one to the other. In this type of system, the power transmitting side and the power receiving side of clutch means engaged at the time of changeover are synchronized to eliminate any disadvantages resulting from engagement under extreme operating conditions. The clutches are normally arranged to be operated by hydraulic pressure. Such a system is shown in U.S. Pat. No. 3,913,325 issued to T. Miyao and T. Sakai entitled "Output Split Type Hydrostatic Transmission." The clutch which is engaged in response to the application of hydraulic pressure is capable of disengaging almost simultaneously with the discharge of hydraulic pressure. By contrast, the clutch which is disengaged under no load is unable to immediately engage even if hydraulic pressure is supplied because the rise of hydraulic pressure is relatively slow. Therefore, if releasing of one of the clutches and engagement of the other clutch are performed simultaneously at the time of changeover of clutches, both clutches are momentarily brought into a released state, inducing a no-load neutral state in the transmission. This causes a sharp increase of rotational speed of the engine, ultimately causing various mechanical problems in both the engine and transmission.